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          异步遍历器
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      </script></div> <div class="theme-vdoing-content content__default"><h1 id="异步遍历器"><a href="#异步遍历器" class="header-anchor">#</a> 异步遍历器</h1> <h2 id="同步遍历器的问题"><a href="#同步遍历器的问题" class="header-anchor">#</a> 同步遍历器的问题</h2> <p>《遍历器》一章说过，Iterator 接口是一种数据遍历的协议，只要调用遍历器对象的<code>next</code>方法，就会得到一个对象，表示当前遍历指针所在的那个位置的信息。<code>next</code>方法返回的对象的结构是<code>{value, done}</code>，其中<code>value</code>表示当前的数据的值，<code>done</code>是一个布尔值，表示遍历是否结束。
</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token punctuation">{</span> value<span class="token operator">:</span> index<span class="token operator">++</span><span class="token punctuation">,</span> done<span class="token operator">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> it <span class="token operator">=</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 0</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 1</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 2</span>
<span class="token comment">// ...</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码中，变量<code>it</code>是一个遍历器（iterator）。每次调用<code>it.next()</code>方法，就返回一个对象，表示当前遍历位置的信息。</p> <p>这里隐含着一个规定，<code>it.next()</code>方法必须是同步的，只要调用就必须立刻返回值。也就是说，一旦执行<code>it.next()</code>方法，就必须同步地得到<code>value</code>和<code>done</code>这两个属性。如果遍历指针正好指向同步操作，当然没有问题，但对于异步操作，就不太合适了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
          <span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">{</span> value<span class="token operator">:</span> index<span class="token operator">++</span><span class="token punctuation">,</span> done<span class="token operator">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码中，<code>next()</code>方法返回的是一个 Promise 对象，这样就不行，不符合 Iterator 协议，只要代码里面包含异步操作都不行。也就是说，Iterator 协议里面<code>next()</code>方法只能包含同步操作。</p> <p>目前的解决方法是，将异步操作包装成 Thunk 函数或者 Promise 对象，即<code>next()</code>方法返回值的<code>value</code>属性是一个 Thunk 函数或者 Promise 对象，等待以后返回真正的值，而<code>done</code>属性则还是同步产生的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token punctuation">{</span>
        value<span class="token operator">:</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token parameter">resolve</span> <span class="token operator">=&gt;</span> <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token function">resolve</span><span class="token punctuation">(</span>index<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
        done<span class="token operator">:</span> <span class="token boolean">false</span>
      <span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> it <span class="token operator">=</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">o</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">o</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">o</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
<span class="token comment">// ...</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><p>上面代码中，<code>value</code>属性的返回值是一个 Promise 对象，用来放置异步操作。但是这样写很麻烦，不太符合直觉，语义也比较绕。</p> <p>ES2018 <a href="https://github.com/tc39/proposal-async-iteration" target="_blank" rel="noopener noreferrer">引入<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>了“异步遍历器”（Async Iterator），为异步操作提供原生的遍历器接口，即<code>value</code>和<code>done</code>这两个属性都是异步产生。</p> <h2 id="异步遍历的接口"><a href="#异步遍历的接口" class="header-anchor">#</a> 异步遍历的接口</h2> <p>异步遍历器的最大的语法特点，就是调用遍历器的<code>next</code>方法，返回的是一个 Promise 对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>asyncIterator
  <span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
  <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span>
    <span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span> value<span class="token punctuation">,</span> done <span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token comment">/* ... */</span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>asyncIterator</code>是一个异步遍历器，调用<code>next</code>方法以后，返回一个 Promise 对象。因此，可以使用<code>then</code>方法指定，这个 Promise 对象的状态变为<code>resolve</code>以后的回调函数。回调函数的参数，则是一个具有<code>value</code>和<code>done</code>两个属性的对象，这个跟同步遍历器是一样的。</p> <p>我们知道，一个对象的同步遍历器的接口，部署在<code>Symbol.iterator</code>属性上面。同样地，对象的异步遍历器接口，部署在<code>Symbol.asyncIterator</code>属性上面。不管是什么样的对象，只要它的<code>Symbol.asyncIterator</code>属性有值，就表示应该对它进行异步遍历。</p> <p>下面是一个异步遍历器的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> asyncIterable <span class="token operator">=</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> asyncIterator <span class="token operator">=</span> asyncIterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

asyncIterator
<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">iterResult1</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>iterResult1<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// { value: 'a', done: false }</span>
  <span class="token keyword">return</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">iterResult2</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>iterResult2<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// { value: 'b', done: false }</span>
  <span class="token keyword">return</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">iterResult3</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>iterResult3<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// { value: undefined, done: true }</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码中，异步遍历器其实返回了两次值。第一次调用的时候，返回一个 Promise 对象；等到 Promise 对象<code>resolve</code>了，再返回一个表示当前数据成员信息的对象。这就是说，异步遍历器与同步遍历器最终行为是一致的，只是会先返回 Promise 对象，作为中介。</p> <p>由于异步遍历器的<code>next</code>方法，返回的是一个 Promise 对象。因此，可以把它放在<code>await</code>命令后面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> asyncIterable <span class="token operator">=</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> asyncIterator <span class="token operator">=</span> asyncIterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">await</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// { value: 'a', done: false }</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">await</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// { value: 'b', done: false }</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">await</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// { value: undefined, done: true }</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>next</code>方法用<code>await</code>处理以后，就不必使用<code>then</code>方法了。整个流程已经很接近同步处理了。</p> <p>注意，异步遍历器的<code>next</code>方法是可以连续调用的，不必等到上一步产生的 Promise 对象<code>resolve</code>以后再调用。这种情况下，<code>next</code>方法会累积起来，自动按照每一步的顺序运行下去。下面是一个例子，把所有的<code>next</code>方法放在<code>Promise.all</code>方法里面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> asyncIterable <span class="token operator">=</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> asyncIterator <span class="token operator">=</span> asyncIterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token punctuation">[</span><span class="token punctuation">{</span>value<span class="token operator">:</span> v1<span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>value<span class="token operator">:</span> v2<span class="token punctuation">}</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">all</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
  asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v1<span class="token punctuation">,</span> v2<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// a b</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>另一种用法是一次性调用所有的<code>next</code>方法，然后<code>await</code>最后一步操作。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">runner</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> writer <span class="token operator">=</span> <span class="token function">openFile</span><span class="token punctuation">(</span><span class="token string">'someFile.txt'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  writer<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  writer<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">await</span> writer<span class="token punctuation">.</span><span class="token function">return</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">runner</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><h2 id="for-await-of"><a href="#for-await-of" class="header-anchor">#</a> for await...of</h2> <p>前面介绍过，<code>for...of</code>循环用于遍历同步的 Iterator 接口。新引入的<code>for await...of</code>循环，则是用于遍历异步的 Iterator 接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> x <span class="token keyword">of</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">// a</span>
<span class="token comment">// b</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>createAsyncIterable()</code>返回一个拥有异步遍历器接口的对象，<code>for...of</code>循环自动调用这个对象的异步遍历器的<code>next</code>方法，会得到一个 Promise 对象。<code>await</code>用来处理这个 Promise 对象，一旦<code>resolve</code>，就把得到的值（<code>x</code>）传入<code>for...of</code>的循环体。</p> <p><code>for await...of</code>循环的一个用途，是部署了 asyncIterable 操作的异步接口，可以直接放入这个循环。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> body <span class="token operator">=</span> <span class="token string">''</span><span class="token punctuation">;</span>

<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span><span class="token punctuation">(</span><span class="token keyword">const</span> data <span class="token keyword">of</span> req<span class="token punctuation">)</span> body <span class="token operator">+=</span> data<span class="token punctuation">;</span>
  <span class="token keyword">const</span> parsed <span class="token operator">=</span> <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span>body<span class="token punctuation">)</span><span class="token punctuation">;</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'got'</span><span class="token punctuation">,</span> parsed<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>req</code>是一个 asyncIterable 对象，用来异步读取数据。可以看到，使用<code>for await...of</code>循环以后，代码会非常简洁。</p> <p>如果<code>next</code>方法返回的 Promise 对象被<code>reject</code>，<code>for await...of</code>就会报错，要用<code>try...catch</code>捕捉。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">try</span> <span class="token punctuation">{</span>
    <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> x <span class="token keyword">of</span> <span class="token function">createRejectingIterable</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">error</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>注意，<code>for await...of</code>循环也可以用于同步遍历器。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> x <span class="token keyword">of</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// a</span>
<span class="token comment">// b</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>Node v10 支持异步遍历器，Stream 就部署了这个接口。下面是读取文件的传统写法与异步遍历器写法的差异。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 传统写法</span>
<span class="token keyword">function</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token parameter">inputFilePath</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> readStream <span class="token operator">=</span> fs<span class="token punctuation">.</span><span class="token function">createReadStream</span><span class="token punctuation">(</span>
    inputFilePath<span class="token punctuation">,</span>
    <span class="token punctuation">{</span> encoding<span class="token operator">:</span> <span class="token string">'utf8'</span><span class="token punctuation">,</span> highWaterMark<span class="token operator">:</span> <span class="token number">1024</span> <span class="token punctuation">}</span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
  readStream<span class="token punctuation">.</span><span class="token function">on</span><span class="token punctuation">(</span><span class="token string">'data'</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token parameter">chunk</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'&gt;&gt;&gt; '</span><span class="token operator">+</span>chunk<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  readStream<span class="token punctuation">.</span><span class="token function">on</span><span class="token punctuation">(</span><span class="token string">'end'</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'### DONE ###'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 异步遍历器写法</span>
<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token parameter">inputFilePath</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> readStream <span class="token operator">=</span> fs<span class="token punctuation">.</span><span class="token function">createReadStream</span><span class="token punctuation">(</span>
    inputFilePath<span class="token punctuation">,</span>
    <span class="token punctuation">{</span> encoding<span class="token operator">:</span> <span class="token string">'utf8'</span><span class="token punctuation">,</span> highWaterMark<span class="token operator">:</span> <span class="token number">1024</span> <span class="token punctuation">}</span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> chunk <span class="token keyword">of</span> readStream<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'&gt;&gt;&gt; '</span><span class="token operator">+</span>chunk<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'### DONE ###'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br></div></div><h2 id="异步-generator-函数"><a href="#异步-generator-函数" class="header-anchor">#</a> 异步 Generator 函数</h2> <p>就像 Generator 函数返回一个同步遍历器对象一样，异步 Generator 函数的作用，是返回一个异步遍历器对象。</p> <p>在语法上，异步 Generator 函数就是<code>async</code>函数与 Generator 函数的结合。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">yield</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">const</span> genObj <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
genObj<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// { value: 'hello', done: false }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，<code>gen</code>是一个异步 Generator 函数，执行后返回一个异步 Iterator 对象。对该对象调用<code>next</code>方法，返回一个 Promise 对象。</p> <p>异步遍历器的设计目的之一，就是 Generator 函数处理同步操作和异步操作时，能够使用同一套接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 同步 Generator 函数</span>
<span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">iterable<span class="token punctuation">,</span> func</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> iter <span class="token operator">=</span> iterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> <span class="token punctuation">{</span>value<span class="token punctuation">,</span> done<span class="token punctuation">}</span> <span class="token operator">=</span> iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>done<span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token function">func</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token comment">// 异步 Generator 函数</span>
<span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">iterable<span class="token punctuation">,</span> func</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> iter <span class="token operator">=</span> iterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> <span class="token punctuation">{</span>value<span class="token punctuation">,</span> done<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">await</span> iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>done<span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token function">func</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><p>上面代码中，<code>map</code>是一个 Generator 函数，第一个参数是可遍历对象<code>iterable</code>，第二个参数是一个回调函数<code>func</code>。<code>map</code>的作用是将<code>iterable</code>每一步返回的值，使用<code>func</code>进行处理。上面有两个版本的<code>map</code>，前一个处理同步遍历器，后一个处理异步遍历器，可以看到两个版本的写法基本上是一致的。</p> <p>下面是另一个异步 Generator 函数的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">readLines</span><span class="token punctuation">(</span><span class="token parameter">path</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> file <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fileOpen</span><span class="token punctuation">(</span>path<span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">try</span> <span class="token punctuation">{</span>
    <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>file<span class="token punctuation">.</span><span class="token constant">EOF</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">yield</span> <span class="token keyword">await</span> file<span class="token punctuation">.</span><span class="token function">readLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    <span class="token keyword">await</span> file<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，异步操作前面使用<code>await</code>关键字标明，即<code>await</code>后面的操作，应该返回 Promise 对象。凡是使用<code>yield</code>关键字的地方，就是<code>next</code>方法停下来的地方，它后面的表达式的值（即<code>await file.readLine()</code>的值），会作为<code>next()</code>返回对象的<code>value</code>属性，这一点是与同步 Generator 函数一致的。</p> <p>异步 Generator 函数内部，能够同时使用<code>await</code>和<code>yield</code>命令。可以这样理解，<code>await</code>命令用于将外部操作产生的值输入函数内部，<code>yield</code>命令用于将函数内部的值输出。</p> <p>上面代码定义的异步 Generator 函数的用法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> line <span class="token keyword">of</span> <span class="token function">readLines</span><span class="token punctuation">(</span>filePath<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>line<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>异步 Generator 函数可以与<code>for await...of</code>循环结合起来使用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">prefixLines</span><span class="token punctuation">(</span><span class="token parameter">asyncIterable</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> line <span class="token keyword">of</span> asyncIterable<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> <span class="token string">'&gt; '</span> <span class="token operator">+</span> line<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>异步 Generator 函数的返回值是一个异步 Iterator，即每次调用它的<code>next</code>方法，会返回一个 Promise 对象，也就是说，跟在<code>yield</code>命令后面的，应该是一个 Promise 对象。如果像上面那个例子那样，<code>yield</code>命令后面是一个字符串，会被自动包装成一个 Promise 对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">fetchRandom</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> url <span class="token operator">=</span> <span class="token string">'https://www.random.org/decimal-fractions/'</span>
    <span class="token operator">+</span> <span class="token string">'?num=1&amp;dec=10&amp;col=1&amp;format=plain&amp;rnd=new'</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token function">fetch</span><span class="token punctuation">(</span>url<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">asyncGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Start'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> result <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fetchRandom</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// (A)</span>
  <span class="token keyword">yield</span> <span class="token string">'Result: '</span> <span class="token operator">+</span> <span class="token keyword">await</span> result<span class="token punctuation">.</span><span class="token function">text</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// (B)</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Done'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> ag <span class="token operator">=</span> <span class="token function">asyncGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
ag<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span>value<span class="token punctuation">,</span> done<span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>上面代码中，<code>ag</code>是<code>asyncGenerator</code>函数返回的异步遍历器对象。调用<code>ag.next()</code>以后，上面代码的执行顺序如下。</p> <ol><li><code>ag.next()</code>立刻返回一个 Promise 对象。</li> <li><code>asyncGenerator</code>函数开始执行，打印出<code>Start</code>。</li> <li><code>await</code>命令返回一个 Promise 对象，<code>asyncGenerator</code>函数停在这里。</li> <li>A 处变成 fulfilled 状态，产生的值放入<code>result</code>变量，<code>asyncGenerator</code>函数继续往下执行。</li> <li>函数在 B 处的<code>yield</code>暂停执行，一旦<code>yield</code>命令取到值，<code>ag.next()</code>返回的那个 Promise 对象变成 fulfilled 状态。</li> <li><code>ag.next()</code>后面的<code>then</code>方法指定的回调函数开始执行。该回调函数的参数是一个对象<code>{value, done}</code>，其中<code>value</code>的值是<code>yield</code>命令后面的那个表达式的值，<code>done</code>的值是<code>false</code>。</li></ol> <p>A 和 B 两行的作用类似于下面的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token function">fetchRandom</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
  <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">result</span> <span class="token operator">=&gt;</span> result<span class="token punctuation">.</span><span class="token function">text</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
  <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">result</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
     <span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
       value<span class="token operator">:</span> <span class="token string">'Result: '</span> <span class="token operator">+</span> result<span class="token punctuation">,</span>
       done<span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
     <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>如果异步 Generator 函数抛出错误，会导致 Promise 对象的状态变为<code>reject</code>，然后抛出的错误被<code>catch</code>方法捕获。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">asyncGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'Problem!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">asyncGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">err</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Error: Problem!</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>注意，普通的 async 函数返回的是一个 Promise 对象，而异步 Generator 函数返回的是一个异步 Iterator 对象。可以这样理解，async 函数和异步 Generator 函数，是封装异步操作的两种方法，都用来达到同一种目的。区别在于，前者自带执行器，后者通过<code>for await...of</code>执行，或者自己编写执行器。下面就是一个异步 Generator 函数的执行器。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">takeAsync</span><span class="token punctuation">(</span><span class="token parameter">asyncIterable<span class="token punctuation">,</span> count <span class="token operator">=</span> <span class="token number">Infinity</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> result <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> iterator <span class="token operator">=</span> asyncIterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">while</span> <span class="token punctuation">(</span>result<span class="token punctuation">.</span>length <span class="token operator">&lt;</span> count<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> <span class="token punctuation">{</span>value<span class="token punctuation">,</span> done<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">await</span> iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>done<span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    result<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> result<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，异步 Generator 函数产生的异步遍历器，会通过<code>while</code>循环自动执行，每当<code>await iterator.next()</code>完成，就会进入下一轮循环。一旦<code>done</code>属性变为<code>true</code>，就会跳出循环，异步遍历器执行结束。</p> <p>下面是这个自动执行器的一个使用实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> <span class="token string">'a'</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token string">'b'</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token string">'c'</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token keyword">return</span> <span class="token keyword">await</span> <span class="token function">takeAsync</span><span class="token punctuation">(</span><span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">result</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ['a', 'b', 'c']</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>异步 Generator 函数出现以后，JavaScript 就有了四种函数形式：普通函数、async 函数、Generator 函数和异步 Generator 函数。请注意区分每种函数的不同之处。基本上，如果是一系列按照顺序执行的异步操作（比如读取文件，然后写入新内容，再存入硬盘），可以使用 async 函数；如果是一系列产生相同数据结构的异步操作（比如一行一行读取文件），可以使用异步 Generator 函数。</p> <p>异步 Generator 函数也可以通过<code>next</code>方法的参数，接收外部传入的数据。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> writer <span class="token operator">=</span> <span class="token function">openFile</span><span class="token punctuation">(</span><span class="token string">'someFile.txt'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
writer<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 立即执行</span>
writer<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 立即执行</span>
<span class="token keyword">await</span> writer<span class="token punctuation">.</span><span class="token function">return</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 等待写入结束</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>openFile</code>是一个异步 Generator 函数。<code>next</code>方法的参数，向该函数内部的操作传入数据。每次<code>next</code>方法都是同步执行的，最后的<code>await</code>命令用于等待整个写入操作结束。</p> <p>最后，同步的数据结构，也可以使用异步 Generator 函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token parameter">syncIterable</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">const</span> elem <span class="token keyword">of</span> syncIterable<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> elem<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，由于没有异步操作，所以也就没有使用<code>await</code>关键字。</p> <h2 id="yield-语句"><a href="#yield-语句" class="header-anchor">#</a> yield* 语句</h2> <p><code>yield*</code>语句也可以跟一个异步遍历器。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">yield</span> <span class="token string">'a'</span><span class="token punctuation">;</span>
  <span class="token keyword">yield</span> <span class="token string">'b'</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token number">2</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// result 最终会等于 2</span>
  <span class="token keyword">const</span> result <span class="token operator">=</span> <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">gen1</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>gen2</code>函数里面的<code>result</code>变量，最后的值是<code>2</code>。</p> <p>与同步 Generator 函数一样，<code>for await...of</code>循环会展开<code>yield*</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> x <span class="token keyword">of</span> <span class="token function">gen2</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// a</span>
<span class="token comment">// b</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></div></div> <div class="page-slot page-slot-bottom"><!-- 横向自适应 -->
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